_getconftestmodules: use functools.lru_cache

Also renames `_path2confmods` to `_dirpath2confmods` for clarity (it is
expected to be a dirpath in `_importconftest`).

Uses an explicit maxsize, since it appears to be only relevant for a
short period [1].

1: pytest-dev#4237 (comment)

src/_pytest/config/__init__.py

@@ -213,7 +213,7 @@ def __init__(self):
         self._conftest_plugins = set()
 
         # state related to local conftest plugins
-        self._path2confmods = {}
+        self._dirpath2confmods = {}
         self._conftestpath2mod = {}
         self._confcutdir = None
         self._noconftest = False
@@ -384,31 +384,29 @@ def _try_load_conftest(self, anchor):
                 if x.check(dir=1):
                     self._getconftestmodules(x)
 
+    @lru_cache(maxsize=128)
     def _getconftestmodules(self, path):
         if self._noconftest:
             return []
 
-        try:
-            return self._path2confmods[path]
-        except KeyError:
-            if path.isfile():
-                directory = path.dirpath()
-            else:
-                directory = path
-            # XXX these days we may rather want to use config.rootdir
-            # and allow users to opt into looking into the rootdir parent
-            # directories instead of requiring to specify confcutdir
-            clist = []
-            for parent in directory.realpath().parts():
-                if self._confcutdir and self._confcutdir.relto(parent):
-                    continue
-                conftestpath = parent.join("conftest.py")
-                if conftestpath.isfile():
-                    mod = self._importconftest(conftestpath)
-                    clist.append(mod)
-
-            self._path2confmods[path] = clist
-            return clist
+        if path.isfile():
+            directory = path.dirpath()
+        else:
+            directory = path
+
+        # XXX these days we may rather want to use config.rootdir
+        # and allow users to opt into looking into the rootdir parent
+        # directories instead of requiring to specify confcutdir
+        clist = []
+        for parent in directory.realpath().parts():
+            if self._confcutdir and self._confcutdir.relto(parent):
+                continue
+            conftestpath = parent.join("conftest.py")
+            if conftestpath.isfile():
+                mod = self._importconftest(conftestpath)
+                clist.append(mod)
+        self._dirpath2confmods[directory] = clist
+        return clist
 
     def _rget_with_confmod(self, name, path):
         modules = self._getconftestmodules(path)
@@ -449,8 +447,8 @@ def _importconftest(self, conftestpath):
             self._conftest_plugins.add(mod)
             self._conftestpath2mod[conftestpath] = mod
             dirpath = conftestpath.dirpath()
-            if dirpath in self._path2confmods:
-                for path, mods in self._path2confmods.items():
+            if dirpath in self._dirpath2confmods:
+                for path, mods in self._dirpath2confmods.items():
                     if path and path.relto(dirpath) or path == dirpath:
                         assert mod not in mods
                         mods.append(mod)